1. Field of the Invention
This application relates to the field of vibration analysis and more particularly to the field of performing vibration analysis for the purpose of providing device adjustments that reduce vibrations.
2. Description of Related Art
Rotors which propel helicopters and other propeller-driven aircraft induce vibrations in the structure supporting the rotor. The vibrations occur at frequencies that correspond to the shaft rotation rate and harmonics thereof. The vibrations may result in a structural damage, crew fatigue, and ultimately become one of the factors limiting the maximum forward speed of the aircraft. Similar types of vibrations are produced by fans and compressors and fixed installations as well as by marine propellers.
A primary source of the vibration problem is non-uniform air loads on the blades, although mass imbalance is not uncommon. Aerodynamic anomalies, however, tend to develop recurrently due to blade wear, damage, deformation, etc. The aerodynamic and mass and stiffness distribution anomalies have often been called xe2x80x9ctracking faultsxe2x80x9d, since a primary observable feature of the uneven airloads or mass distribution is it tendency for the blades to flap and/or deflect unevenly, and thus follow different xe2x80x9ctracksxe2x80x9d. The troublesome manifestation of the aerodynamic and mass imbalance, however, is usually the 1/rev and n/rev vibrations and not the track deviations themselves.
It is possible to modify the vibration characteristics of a helicopter by xe2x80x9crotor trimmingxe2x80x9d, which involves adjusting the weight of the blades at the hub, the tab setting at one or more blades, and the adjustment on the pitch rods. However, determining the effect of each of these adjustments may be difficult because the interdependence of the adjustments. This interdependence may be the source of some difficulty with trial and error methods of rotor trimming, which may allow variation of only one type of adjustment at a time. One set of adjustments may be thrown out of kilter by a subsequent step in the process, requiring repetitive adjustments which may or may not converge to an acceptable state.
Some helicopter rotor trim balancing methods rely, at least in part, upon making the track of each blade identical using, for example, known optical methods. Optical methods, however, employ bulky equipment which relies upon an operator in the co-pilot seat and procedures which require considerable flight time. Furthermore, optical methods cannot always xe2x80x9cseexe2x80x9d the blades during a complete revolution and thus cannot be expected to achieve perfect aerodynamic trim. Thus, it is desirable to trim the rotor without having to resort to optical tracking. Fortunately, mechanically balancing the rotors to reduce vibrations often has the desirable effect of improving the rotor track.
Mechanical balancing of rotors with mass imbalance may, in some cases, be performed with a single accelerometer and a shaft-phase reference sensor. However, uneven air loads may not be fully diagnosed and corrected with such a technique. Other techniques used to perform the rotor smoothing function may rely upon optical tracking in conjunction accelerometers. Known rotor smoothing systems, however, process vibration data in such a way that there may be an inherent ambiguity in the interpretation of the signatures. The ambiguity comes about because, in many cases, the number of channels processed simultaneously is inadequate to fully separate translational and rotational acceleration components at a given point. Thus, the motion of the helicopter (and in particular the rotor support) in response to a rotor anomalies may be incompletely specified. Furthermore, some systems may not deduce the corrections needed from the Fourier coefficients related to each anomaly.
U.S. Pat. No. 4,937,758, which is incorporated herein by reference, attempts to address these problems by disclosing techniques that take into account the interdependence of the adjustments. However, the system disclosed therein suffers from drawbacks, including difficulties in adequately relating different types of adjustments to each other and difficulties associated with specifying adjustments that do not take into account acceptable granularity and quantization of the adjustments. Furthermore, it is also desirable to minimize the risk associated with a proposed adjustment where the risk corresponds to the size of the adjustment when compared to the maximum allowable adjustment of that type (i.e., it is more risky to make a large adjustment than a small one). In addition, in some instances it is desirable to be able to correlate risks associated with different types of adjustments (e.g., compare the risk associated with an N ounce hub weigh adjustment with the risk associated with an M degree tab bend).
According to the present invention, determining adjustments to decrease vibrations caused by rotating blades includes receiving a plurality of vibration values corresponding to vibrations caused by the blades, providing coefficient of vibration data that corresponds to effects on vibration caused by adjustments to the blades, where the adjustments include at least two different types of adjustments having different units of measure, applying the coefficient of vibration data to the vibration values, and solving for possible values for the adjustments that may be applied to the blades, where the possible values for the adjustments results in the vibrations being less than a predetermined value and where the different types of adjustments are normalized according to a maximum amount of adjustment for each type of adjustment. The predetermined value may correspond to an expected value of all errors. The predetermined value may be increased in order to decrease the values for the adjustments that may be applied to the blades. The adjustments may include at least one of hub weights, blade tabs, PCR adjustments, and blade tip weight adjustments. Measuring vibrations may include obtaining signals from a plurality of accelerometers. Determining adjustments to decrease vibrations caused by rotating blades may also include performing an FFT on the vibration data, where the data indicating effects on vibration is provided in the frequency domain. Applying the data to the vibration values may be performed in the frequency domain. The vibration values, the coefficient of vibration data, and the adjustments may all be matrixes. The vibration values may include a first matrix, u, corresponding to measured vibrations and a second matrix, v, corresponding to desired vibrations. Solving for possible values for the adjustments may use the equation: v=u+Xxc2x7a, where a represents the adjustment and X represents the coefficient of vibration data. Solving for possible values of the adjustments may include replacing X with Qxc2x7R, where QTxc2x7Q equals the identity matrix, I and R is an upper triangular matrix. Solving for possible values of the adjustments may include evaluating |QTxc2x7u+Rxc2x7a|. A value for a may be determined using a numeric technique to find a local minimum for aTxc2x7Exc2x7a subject to a constraint that |QTxc2x7u+Rxc2x7a| is less than the predetermined value, wherein E is a diagonal matrix having elements corresponding to a predetermined scaling amount for each type of adjustment. The predetermined scaling amount may correspond to an inverse of the maximum amount of adjustment for each type of adjustment.
According further to the present invention, computer software that determines adjustments to decrease vibrations caused by rotating blades includes executable code that accesses data corresponding to a plurality of vibration values corresponding to vibrations caused by the blades, executable code that accesses coefficient of vibration data that corresponds to effects on vibration caused by adjustments to the blades, wherein the adjustments include at least two different types of adjustments having different units of measure, executable code that applies the coefficient of vibration data to the vibration values, and executable code that solves for possible values for the adjustments that may be applied to the blades, where the possible values for the adjustments results in the vibrations being less than a predetermined value and wherein the different types of adjustments are normalized according to a maximum amount of adjustment for each type of adjustment. The predetermined value may correspond to an expected value of all errors. The predetermined value may be increased in order to decrease the values for the adjustments that may be applied to the blades. The adjustments may include at least one of hub weights, blade tabs, PCR adjustments, and blade tip weight adjustments. Data corresponding to a plurality of vibration values may include data corresponding to signals from a plurality of accelerometers. The computer software may also include executable code that performs an FFT on the vibration data, where the data indicating effects on vibration is provided in the frequency domain. Applying the coefficient of vibration data to the vibration values may be performed in the frequency domain. The vibration values, the coefficient of vibration data, and the adjustments may all be matrixes. The vibration values may include a first matrix, u, corresponding to measured vibrations and a second matrix, v, corresponding to desired vibrations. Executable code that solves for possible values for the adjustments may use the equation: v=u+Xxc2x7a, where a represents the adjustment and X represents the coefficient of vibration data. Executable code that solves for possible values of the adjustments may include executable code that replaces X with Qxc2x7R, where QTxc2x7Q equals the identity matrix, I and R is an upper triangular matrix. Executable code that solves for possible values of the adjustments may evaluate |QTxc2x7u+Rxc2x7a|. A value for a may be determined using a numeric technique to find a local minimum for aTxc2x7Exc2x7a subject to a constraint that |QTxc2x7u+Rxc2x7a| is less than the predetermined value, wherein E is a diagonal matrix having elements corresponding to a predetermined scaling amount for each type of adjustment. The predetermined scaling amount may correspond to an inverse of the maximum amount of adjustment for each type of adjustment.
According further to the present invention, an apparatus for determining adjustments to reduce blade vibrations, includes means for receiving a plurality of vibration values corresponding to vibrations caused by the blades, means for accessing coefficient of vibration data that corresponds to effects on vibration caused by adjustments to the blades, where the adjustments include at least two different types of adjustments having different units of measure, means for applying the coefficient of vibration data to the vibration values, and means for solving for possible values for the adjustments that may be applied to the blades, where the possible values for the adjustments results in the vibrations being less than a predetermined value and where the different types of adjustments are normalized according to a maximum amount of adjustment for each type of adjustment. The means for solving may be fixedly attached to a helicopter or may be portable. The means for receiving a plurality of vibration values may include a plurality of accelerometers.
According further to the present invention, determining blade adjustments includes applying coefficient of vibration data to measured vibration values, where the coefficient of vibration data corresponds to effects on vibration caused by at least two different types of blade adjustments having different units of measure that are normalized according to a maximum amount of adjustment for each type of adjustment and determining a set of values for the adjustments that result in the vibrations being less than a predetermined value.